Transformer-coupled emitter-follower oscillator



1m31, 1967 1 A. PYATT 3,302,131

TRANSFORMER-COUPLED EMITTER-FOLLOWER OSCILLATOR Filed May 9, 1966 fil/ U AW/P'A/c 4. ,Q1/477 ,Q5 INVENTOR.

United States Patent O 3,392,131 TRANSFORMER-COUPLED EMITTER-FULLWER SCILLATOR Lawrence A. Pyatt, Grange, Calif., assigner of twentythree and three-fourths percent to Lawrence A. Pyatt and Bernard Farmen, both of Long Beach, Calif.,

twenty-three and three-fourths percent to Charles J.

Link, Long Beach, Calif., twenty-three and threefourths percent to Harmon G. Scoville, Garden Grove,

Calif., and five percent to Robert W. Thompson, Riverside, Calif.

Filed May 9, 1966, Ser. No. 548,760 11 Claims. (Cl. 331-112) This application is a continuation-in-part of my prior copending application Serial No. 371,511 iiled June l, 1964, which was permitted to become abandoned subsequent to the tiling of this application. That earlier application represented an improvement over a still earlier copending application Serial No. 858,482 filed December 9, 1959.

The present invention is a transformer-coupled emitterfollower oscillator circuit in which a step-up transformer is coupled between the emitter and base of the transistor.

One primary object and purpose of the invention is to provide transistor oscillator circuits having unique and improved characteristics.

Another separate and distinct object of the invention is to provide novel sonic or ultrasonic generators incorporating a transistor oscillator circuit.

The objects and advantages of the invention will be more fully understood from the following description considered in conjunction with the accompanying drawing, wherein:

FIGURE 1 illustrates one form of ultrasonic generator provided in accordance with the invention;

FIGURE 2 illustrates the presently preferred form of ultrasonic generator in accordance with the invention;

FIGURE 2(a) shows voltage waveforms in the circuit of FIGURE 2; and

FIGURE 3 illustrates an oscillator circuit in accordance with the invention, which supplies electrical output energy to a load.

Reference is first made to FIGURE 1 illustrating one form of ultrasonic generator in accordance with the invention. A transistor Q1 of the P-N-P type has its emitter connected to terminal 1 of primary winding 11 of step-up transformer T1, while its collector is connected to the negative terminal of a D.C. power supply identified as Eb. The positive terminal of Eb is connected to terminal 2 of the transformer primary 11. A frequency determining capacitor C1 has one plate connected to the base of transistor Q1, while its other plate is connected to terminal 3 of the transformer secondary winding 12. The polarity marks associated with the transformer indicate that terminal 1 of primary winding 11 and terminal 3 of secondary winding 12 are of the same voltage polarity, i.e., when one is going positive the other is also going positive. A frequency determining inductance coil L1 has one of its ends connected to terminal 3 of transformer secondary winding 12 while its other end is grounded. The collector of transistor Q1 is also grounded. A crystal transducer F is connected between terminal 4 of the transformer secondary winding 12 and ground. Crystal transducer F is mechanically coupled to an ultrasonic load 13.

Additional elements included in the circuit of FIGURE 1 are a capacitor C2 -coupled between the base and emitter of transistor Q1, and a resistor R1 coupled between the base and collector. The circuit will operate very satisfactorily without these elements, as was illustrated in patent application Serial No. 858,482, led December 9, 1959, and R1 and C2 are added to the circuit primarily lCe in order to provide temperature compensation fory the transistor.

The crystal transducer F may, for example, be a barium titanate crystal, but is preferably a PZT type crystal sold by Clevite Corporation under the designation CV4. The crystal F is capable of oscillating at any one of several primary oscillation frequencies, such as 40, 80, and kilocycles, and is characterized by the generation of a substantial amount of ultrasonic energy, when suitably loaded. The ultrasonic load 13 may, for example, consist of a body of water, when the oscillator is incorporated in underwater communication equipment; or it may consist of a cleaning solution; or it may consist of an abrasive compound used for drilling purposes; to mention only a few examples.

The operation of the oscillator circuit is generally as follows. Current ow through the emitter-collector path of the transistor induces a voltage across the transformer primary winding 11, and consequently a stepped-up voltage across the secondary winding 12. Capacitor C1 and inductance L1 act both as a resonant circuit, and as a feedback circuit, and receive energy from the transformer secondary winding 12 and in turn control the voltage of the base of the transistor. Due to the emitter-follower circuit arrangement the emitter voltage follows the base voltage quite closely, both in the positive and negative portions of the cycle. The base is driven to saturation during the negative portion of each voltage cycle, producing sufficient base current to clip the negative voltage peak and to clamp the base voltage, making a D.C. bias unnecessary. The circuit operation is therefore in Class C. The base is driven to cut-off during the positive portion of the cycle.

The crystal F, as previously mentioned, is capable of oscillating at any one of several primary oscillation frequencies, and the frequency at which it in fact does oscillate in the circuit of FGURE 1 is determined by the resonant frequency of capacitor C1 and inductor L1. This resonant frequency is chosen so as to coincide, or nearly coincide, with one of the primary oscillation frequencies ofthe crystal.

Capacitor C2 is chosen ten to twenty times as large as capacitor C1, and bypasses the -base-to-emitter interelectrode capacitance of Q1. C2 therefore serves as a temperature control feedback circuit. When the circuit is first energized the base is grounded through R1, which ensures initial conduction and hence the positive initiation of oscillation.

In the circuit of FIGURE 1 the step-up voltage ratio of transformer T1 may, for example, be l0() to l when using a barium titanate crystal having primary oscillation modes at 40, 20 and 10 kilocycles, or 10 to l when using a CV-4 crystal having primary modes at 160, 80 and 40 kc.

In the circuit of FIGURE 1 the positions of capacitor C1 and inductor L1 may be reversed, and it is then necessary to also reverse the polarity of connection of the tranformer secondary winding 12. Circuit operation is then essentially the same as in the configuration illustrated.

An interesting characteristic of the circuit of FIGURE l is that it has two separate means for determining the oscillation frequency; the resonant circuit provided by C1 and L1, and the oscillation frequency of the crystal transducer F. Where the circuit is used for underwater communications it has been found that the ultrasonic load on the crystal transducer F remains essentially constant, with relatively minor variations due to changes in water temperature. The relatively small changes in the load on the crystal transducer F create a certain tendency to change the oscillation frequency of the oscillator circuit, but not so great as to upset the basic frequency control that is established by L1 and C1. The circuit may be operated off the resonance frequency Iof the crystal transducer, land either L1 or C1 may be made variable so as to permit selection of a desired operating frequency. The Voltage swing across the crystal transducer is many times greater than the voltage swing at the transistor base, and substantial output power is delivered to the load.

When R1 is omitted from the circuit of FIGURE 1 the base is initially grounded through C1 and the plate capacitance of the crystal transducer F, thus again ensuring positive initiation of oscillation.

The novel features of the present invention will now be pointed out with reference to the circuit of FIGURE 1.

The transistor is connected in the circuit in the well known emitter-follower configuration, in which the voltage input to the transistor is the voltage between base Vand collector, while the voltage output is that appearing between emitter and collector. In the emitter-follower circuit configuration, as is well known, the 4gain in voltage is less than unity, being typically about 0.98. Conventional oscillator circuits, as is well known, include an amplifier which has a voltage gain signicantly greater than one. The amplified voltage signal thus produced in a conventional oscillator has sufficient magnitude so that a portion of it may be returned to the input (through the feedback circuit of the oscillaor) so as to maintain the oscillations, and the amplified voltage signal is also `available for supplying energy to a load. The emitterfollower circuit is not conventionally used in oscillators for the obvious reason that its Voltage gain being less than 1, i.e., there being actually a voltage loss, a feedback circuit which returns part of the output voltage to the input circuit will not suiiice for maintaining the oscillations.

The Lowe Patent No. 2,972,116-illustrates a particular circuit configuration for an emitter-follower transistor oscillator. It is not clear from that patent how oscillations are obtained or maintained, since no mechanism for feeding back energy from the output circuit to the input circuit is illustrated. However, as explained in that patent, the critical adjustment of two anti-resonant circuits, one connected in the input circuit of the transistor yand the other connected in the output circuit, must be lmaintained in a certain particular manner in order to permit oscillations to exist at all. The Lowe oscillator is intended for use as a control device, and would be incapable of supplying a significant amount of output power to a load.

Whenever significant output power is to be delivered to a load, it is necessary to maintain efiicient conditions both for generating the power in the first instance and also for transferring it effectively from the generating circuit to the load circuit. A fundamental principle of electrical engineering is that, when purely resistive circuits are involved, the maximum power transfer is achieved when the resistance of the load circuit is exactv 1y equal to the resistance of the source or generator circuit. When reactance elements, either inductive or capactive, are included in the circuit, or when the circuit while not specifically containing either an inductor or a capacitor element is nevertheless characterized by inductive or capacitive properties, the rule for maximum power transfer is then different. In that situation the gener-al principle is that the resistive impedance of the load circuit must be maintained equal to the resistive impedance of the source or generator circuit, and the reactance value of the load circuit at the operating frequency must be equal in value and opposite in kind or quality to the reactance of the source or generating circuit. For example, if the internal impedance of the generating circuit `at the particular operating frequency is 100 ohms resistance and 200 ohms inductive reactance, then the load circuit should be provided with 100 ohms resistance and 2010 ohms capacitive reactance in order to achieve maximum power transfer.

An inherent problem of effectively utilizing the emitterfollower circuit configuration for transistors is that the output circuit has a very low impedance. Load circuits usually have a high impedance, making efficient power transfer impossible when driven by a l-ow impedance source. Particularly is this true in the case of a crystal transducer as the load. The impedance of the transistor emitter is typically of the order of 500 ohms, while the input impedance of the crystal transducer is ordinarily many thousands of ohms.

In order to provide la useful emitter-follower oscillator, capable of delivering substantial output power, it is therefore necessary to provide means for converting la normal voltage loss of the circuit into -a voltage gain; to provide means for efficiently matching the impedance of the transistor to the impedance of a loa-d circuit; and to provide suitable means for feeding back a portion of the output voltage in a proper phase relationship to the input circuit. In accordance with the present invention all of these requirements are achieved by the circuit of FlGURE 1. The transformer primary winding 11 has a relatively few number of turns and is therefore characterized by a low impedance. All of the current through the low-impedance emitter flows through the matched impedance of the primary winding 11, thus achieving efficient power transfer in this, the output circuit of the transistor. The secondary winding 12 of the transformer has la high impedance, which is coupled both to the high input impedance of the crystal transducer F, and also t0 the high base impedance of the transistor. Thus, there is also efficient impedance matching in the circuit which delivers power to the crystal transducer and hence to the ultimate Iload 13.

Reference is now made to the circuit of FIGUREKZ which is generally similar to the circuit of FIGURE 1. It will be noted, however, that the temperature compensation elements R1 and C2 Iare not included, and also that the frequency determining elements L1 and C1 are omitted. The remaining circuit elements are connected in the same manner -as before, with the feedback line from the upper terminal of secondary winding 12 being connected directly to the transistor base. The circuit of FIGURE 2 also illustrates a particular form of power supply circuit 15, for supplying the necessary operating potential.

The preferred circuit values and operating conditions for the circuit of FIGURE 2 are more specifically described as follows. Transistor Q1 is of the P-N-P type, germanium power transistor 2N514B. The transformer T1 has a step-up ratio of 10 to '1, with ten turns in the prim-ary winding and turns in the secondary winding. The resistance of the secondary winding is kept below 0.1 ohm. The transformer incorporates a toroidal core made of powdered iron having high permeability with substantially no hysteresis loss. The crystal transducer is type CV4, with primary resonance modes of 160, 80, and 40 kilocycles; being of the PZT type with 1/2-inch length and l-inch diameter. The series resistance of this transducer at resonance Iis approximately 70() ohms, and its fixed plate-to-plate capacitance is approximately picofarads. The power supply circuit 15 as illustrated incorporates a full wave rectifier circuit and produces a D.C. output Voltage of 22 volts, which is only partially filtered by a shunt capacitor. The ripple voltage of approximately 10 volts has no adverse effect upon the operation of the oscillator circuit. In the presently preferred application 4of the circuit of FIGURE 2 the ultrasonic load 13 consists of a pressurized Freon solution which is used ifor ultrasonic cleaning purposes; the crystal transducer F is simply immersed in this solution and transmits sonic energy by means of direct mechanical contact with the 'liquid bath.

One problem involved in the circuit of FIGURE 2 is that of establishing the desired oscillation frequency in the first instance. Since the frequency determining elements L1 and C1 of FIGURE 1 are not included in FIGURE 2,

the frequency must be determined by other means, or must be left to chance. It will be noted that the problem is not one of establishing oscillation, because the plate-to-plate capacitance of tnansducer F is initially uncharged and effectively grounds the base of the transistor. Upon energizing the circuit, therefore, a heavy initial current flows through primary winding 11 and through the emitter of the transistor, this heavy initial current being possibly of the order of 1,000 amperes. The voltage induced in the secondary winding I2 of the transformer T1 insures a feedback signal to the transistor base, and hence oscillation, but this oscillation could possibly be initiated at any one of the several resonance modes of the crystal transducer F. It is significant that the plate-to-pliate capacitance of the transducer F is a function of the physical geometry of the plates, and is substantially constant for all oscillation frequencies of the transducer. It has been found that by appropriate selection, by trial and error, of the inductance value of the secondary winding 12 of the transformer, this inductance value together with the inherent plate-to-plate capacitance of transducer F will establish an oscillating frequency which corresponds to a desired one of the resonance modes of the transducer. In other words, the transformer and its secondary winding lare so selected that, at the desired resonance frequency, the net inductance value of the second-ary winding, after allowing for shunt stray capacitance as well as the effective inductance that is subtracted due to the loaded primary winding being coupled to the secondary winding, has a reactance value which yis substantially equal to the reactance of the approximately 150 picofarads capacitance of the transducer.

There is another problem involved in the successful .operation of the circuit of FIGURE 2, which is always present when driving a crystal transducer as a means for delivering mechanical energy to a load. This problem arises from the fa-ct that even at its resonance frequency the transducer does not exhibit a purely resistive impedance, since the plate-to-plate capacitance is always pressent. At any one -of its primary resonance modes the equivalent circuit of the transducer F is essentially equal to 700 ohms resistance, coupled in parallel with 150 `picofarads capacitance. The inherent capability of the crystal, as a result of its vibrations, to exhibit either an inductive or capacitive reactance, is effectively coupled in series with the effective resistance of the device; therefore, when the crystal is vibrating at a frequency other than resonance it has an equivalent circuit consisting of a reactance in series with the approximately 700 ohms resistance, with the 150 piccfarads capacitance coupled in parallel with that series combination. For the purpose of maximum power transfer to it the transducer can be yoperated at a frequency other than resonance, and in that event its effective reactance could be balanced out by its plate-to-plate capacitance. That does not, however, represent the most efficient or desirable operating condition for the transducer. The most efficient and desirable operating condition for the transducer is exactly at resonance. Therefore, it is desired that the transducer be coupled to a driving circuit which exhibits .an inductive reactance, of such magnitude as to counterbalance the plate-to-plate capacitance of the transducer.

As previously indicated, the transformer T1, and its secondardy winding in particular, are selected so as to provide an inductive reactance which, at the desired oscillation frequency, will be effectively equal to the capacitive reactance of the plate-to-plate capacitance of the transducer. It is therefore seen that, by making this selection of the circuit components, two important objectives are achieved at the same time: initial oscillation of the circuit at the desired frequency, rather than one of the undesired frequencies, is assured; and maximum power transfer into the crystal transducer is also assured.

FIGURE 2(a) is an approximate representation of two of the voltage waveforms of the circuit of FIGURE 2.

The upper waveform designated eb is the Voltage wave that appears on the base of the transistor. During the negative portion of the cycle, for about or more, the base voltage is approximately minus 4.0 volts, and the entire transistor is then operating at saturation. The emitter voltage, not illustrated, has at this time a value of approximately minus 3.9 volts. The base voltage then rapidly switches to a positive value of approximately plus 75 volts, representing the cutoff voltage at which neither of the junctions of the transistor is conductive. This cutoff condition is maintained for approximately 160 of the cycle. The values illustrated are for an operating frequency of 80 kilocycles, where the period of one cycle is 12.5 microseconds and the total switching time of the circuit, for switching in both the positive and the negative directions, requires about 1 microsecond per cycle. The conduction of current through the base both clips and clamps the base voltage, making D.C. biasing voltage unnecessary. As shown in FIG- URE 2(a) the quiescent or average voltage of the base is approximately plus 35 volts. In the lower part of FIGURE 2(a) is illustrated the voltage ef appearing across the crystal transducer F. The Voltage swing across the transducer is approximately ten times the magnitude of the voltage swing on the base. The voltage ef as illustrated in FIGURE 2(41) is measured on the upper plate of the transducer F, where it is approximately out of phase with the base voltage. However, although not clearly indicated in FIGURE 2(a), the changes in voltage on the transducer occur one or two degrees ahead of the corresponding changes in the base voltage. The reason for this is, that in the circuit of FIGURE 2 the transducer F is the sole frequency determining element, and when it starts to switch its own condition, it then drives the transistor to switch in correspondence therewith. The switching action of the transistor necessarily involves a time delay which, in the particular illustration, is about one or two degrees.

In its preferred application to ultrasonic cleaning, the circuit of FIGURE 2 does not have a really stable oscillating frequency, but rather, its oscillation frequency varies from time to time with changes in the load conditions presented by the ultrasonic cleaning bath. This change in frequency is desirable, however, because it serves to maintain the oscillation of the transducer exactly at its resonance frequency. When its resonance frequency is changed by changes in the load to which the transducer is coupled, the oscillating frequency of the transducer changes accordingly; its most efficient operation is maintained at al1 times. This variable frequency characteristic, while it would be undesirable in many other applications, is of great value in this particular application. The circuit of FIGURE 2 is capable of delivering approximately 1,000 watts of energy to an ultrasonic cleaning bath.

Reference is now made to FIGURE 3, illustrating another oscillator circuit in accordance with the invention. This circuit is generally the same as the circuit `of FIG- URE 2, with certain changes as follows. A resistance R2 constitutes the electrical load on the circuit, and is connected across the emitter-collector terminals of the transistor. The crystal G is not a crystal transducer, for providing a mechanical energy output, but is a conventional piezoelectric -or quartz crystal used solely for frequency determination. Due to the different characteristics of the crystal it is necessary to provide special means for initiating the circuit oscillation, at the desired frequency, and this is done by a shunt resistance R3 thatA is coupled in parallel with the crystal G and is selectively switched into the circuit by means of a switch 18. Switch 18 is opened after oscillation has been initiated. The crystal G may for example be a quartz crystal having an oscillation frequency of 96 kilocycles, while resistor R3 has a value of 2 megohms.

While the load is illustrated as being a pure resistance R2, either inductive or capacitive loads may be efficiently driven with the circuit of FlGURE 3. The circuit provides a high power output, and at a very stable frequency which is unaffected by load variations. The particular value of this circuit, aside from its ecient generation of high output power, is the fact that it maintains a stable frequency 'without the necessity for a special buffer stage between the transistor and its load.

It will therefore be seen that the novel emitter-follower oscillator circuits of the present invention may be adapted to varying requirements, including variable frequency, selected as desired; frequency which follows the changes of the load so as to provide maximum power output to a sonic or ultrasonic load; and circuits which maintain a stable frequency in spite of load variations.

The invention has been described in considerable detail in order to comply with the patent laws by providing a full public disclosure of at least one of its forms. However, such detailed description is not intended in any way to limit the broad features or principles of the invention, or the scope of patent monopoly to be granted.

Having described the invention, what is claimed as new in support of Letters Patent is:

1. A transformer-coupled emitter-feedback transistor oscillator circuit comprising, in combination:

a transistor having base, emitter, and collector electrodes;

a step-up transformer having a primary winding and a secondary winding, and having one end of its primary Winding coupled to said emitter;

power supply means biasing the other end of said primary winding relative to sa-id collector;

a frequency-determining crystal coupled between said collector and one end of said secon-diary winding, the other end of said secondary lwinding being coupled to said base;

and means for coupling output energy from said oscillator to a load.

2. The oscillator circuit of claim 1 wherein said crystal is a crystal transducer, and said coupling means is operative for coupling mechanical vibrations from said crystal transducer to a load.

3. The oscillator circuit of claim 2 wherein the inductance of s-aid secondary winding is selected so as to substantially offset the input capacitance of said crystal transducer at a selected resonance frequency, thereby ensuring circuit oscillation at the selected resonance frequency as well as maximum power transfer to said crystal transducer.

4. The oscillator circuit of claim 1 wherein said coupling means is coupled between said emitter and said collector.

5. The oscillator circuit of claim y1 wherein said other end of said secondary wind-ing is directly connected to said base, said other end of said secondary winding and said one end of said primary winding being of the same voltage polarity. l

6. The oscillator circuit of claim 1 which add-ition-ally includes lan inductor and a capacitor connected together in a series circuit which interconnects said collector and said lbase, .said other end of said secondary winding being connected to the juncture of said inductor `and capacitor, the resonant frequency of said series circuit being equal to a selected resonance frequency of said crystal.

7. A transformer-coupled emitter-feedback oscillator circuit comprising in combination:

a transistor having base, emitter, and collector electrodes, said base having a high impedance and said emitter having a low impedance;

a step-up transformer having primary and secondary windings, said primary winding being of low impedance comparable to said emitter impedance, said secondary winding being of lhigh impedance comparable to said base impedance;

power supply means connected to said collector, on

end of said primary winding being connected to said emitter and its other end being connected to said power supply means, whereby the entire current generated by said power supply means ows both through said emitter and through said primary winda frequency-determining crystal having a high imped- .ance comparable to said secondary winding impedance, and interconnected between one end of said secondary winding and said collector, the other end of said secondary winding being coupled to said base whereby all of the current supplied to said crystal flows through said secondary winding;

and means for coupling output energy from said oscillator to la load.

8. The oscillator circuit of claim 7 wherein said other end of said secondary winding is directly connected to said base.

9. The oscillator circuit of claim 8 wherein said crystal is a crystal transducer, and said coupling means is operative for coupling mechanical vibrations from said crystal transducer to a load.

10. The oscillator circuit of claim 9 'wherein the inductance of said secondary winding is selected so as to substantially offset the input capacitance of said crystal transducer at a selected resonance frequency, thereby en* suring circuit oscillation at the selected resonance frequency as well as maximum power transfer to said crystal transducer.

11. The oscillator circuit of claim 8 wherein said coupling means is coupled between said emitter and said collector.

No references cited.

ROY LAKE, Primary Examiner.

S. H. GRIMM, Assistant Examiner. 

1. A TRANSFORMER-COUPLED EMITTER-FEEDBACK TRANSISTOR OSCILLATOR CIRCUIT COMPRISING, IN COMBINATION: A TRANSISTOR HAVING BASE, EMITTER, AND COLLECTOR ELECTRODES; A STEP-UP TRANSFORMER HAVING A PRIMARY WINDING AND A SECONDARY WINDING, AND HAVING ONE END OF ITS PRIMARY WINDING COUPLED TO SAID EMITTER; POWER SUPPLY MEANS BIASING THE OTHER END OF SAID PRIMARY WINDING RELATIVE TO SAID COLLECTOR; A FREQUENCY-DETERMINING CRYSTAL COUPLED BETWEEN SAID COLLECTOR AND ONE END OF SAID SECONDARY WINDING, THE OTHER END OF SAID SECONDARY WINDING BEING COUPLED TO SAID BASE; AND MEANS FOR COUPLING OUTPUT ENERGY FROM SAID OSCILLATOR TO A LOAD. 